1. Field of the Invention
The present invention relates to a D.C. (direct current) power supply apparatus for supplying D.C. power to an inverter which converts D.C. power into A.C. (alternating current) power and more particularly relates to improvement of a D.C. power supply apparatus formed of, for example, a thyristor converter, chopper circuit, high power factor converter, or the like.
2. Description of the Prior Art
For supplying the load side with A.C. power of stabilized frequency and voltage characteristics, an inverter for converting D.C. power into A.C. power is generally used. The D.C. power supply apparatus for the inverter is provided on the input side of the inverter to supply a stabilized D.C. output to the inverter.
Those so far most widely used as such D.C. power supply apparatus for inverters are thyristor type rectifier apparatus (thyristor rectifiers), and as an example of the same, there is known the CA type rectifier disclosed in "Technical Reference on GS CA type Thyristor Rectifier", on pages 8 to 9, distributed by Nippon Denchi K.K. (Japan Battery Co., Ltd.).
The conventional D.C. power supply apparatus was, as shown in FIG. 1, made up of a thyristor rectifier circuit as a forward conversion apparatus converting A.C. power into D.C. power of which voltage (current) is controlled at the same time, such as, for example, a thyristor rectifier 1 of the above mentioned CA type, a D.C. reactor 2 provided on the output side of the thyristor rectifier 1 for smoothing the pulsating component in the D.C. current of the output of the thyristor rectifier 1, a storage battery 3 connected in parallel between the D.C. reactor 2 and an inverter as a reverse conversion apparatus (not shown), a voltage detector 4 for detecting the output voltage of the thyristor rectifier 1, a current detector 5 for detecting the output current of the thyristor rectifier 1, an upper limit value of output current generation circuit 6 for generating the upper limit of the output current of the thyristor rectifier 1, a current drooping control circuit 7 in response to the detected signal by the current detector 5 and the set signal by the upper limit value of output current generation circuit 6 for controlling the output current of the thyristor rectifier 1 so that it may not exceed the upper limit, a reference voltage generation circuit 8 for generating the reference value of the output voltage of the thyristor rectifier 1, an output voltage control circuit 9 in response to the outputs of the voltage detector 4, current drooping control circuit 7, and the reference voltage generation circuit 8 for controlling the output voltage of the thyristor rectifier 1 so that it may be held constant, and a gate pulse generation circuit 10 in response to the output of the output voltage control circuit 9 for outputting a predetermined gate pulse.
Operations of the same will be described below. The output voltage and current of the thyristor rectifier 1 converting input A.C. power into D.C. power are detected by their respective detectors 4 and 5 after being smoothed by the D.C. reactor 2. Then, the output voltage control circuit 9 compares the detected voltage value from the voltage detector 4 with the reference value from the reference output voltage generation circuit 8 and sends a signal to the gate pulse generating circuit 10 so that the detected voltage and the reference voltage may become equal.
Meanwhile, the current drooping control circuit 7 applies a signal to the output voltage control circuit 9 so that the output current may be reduced when the detected current value exceeds the reference value from the upper limit value of output current generation circuit 6.
Then, the gate pulse generating circuit 10 generates a gate pulse whose phase is controlled by the same in response to the signal from the output voltage control circuit 9. This gate pulse is applied to the thyristor rectifier 1, whereby the output voltage of the thyristor rectifier 1 is held constant and the output current thereof is maintained below the upper limit value.
In the event of a power failure, the D.C. voltage is supplied from the storage battery 3.
Since the prior art D.C. power supply apparatus for an inverter was constructed as described above, there were problems with the same when applied to a power souce of an inverter for uninterruptible equipment or the like as follows.
(1) Since the D.C. input current to an inverter contains a ripple corresponding to the output current thereof, a so-called electrolytic capacitor is generally used as a smoothing capacitor for passing the ripple current therethrough. But, since the so-called electrolytic capacitor has a short life of only 7-8 years, it has to be replaced rather frequently and this requires much labor and cost.
(2) In the case where a so-called film capacitor is used instead of the so-called electrolytic capacitor, the capacity becomes 1/10 or less in view of the economy achievable and the space occupied thereby, and thus, the ripple in the D.C. voltage becomes larger. Therefore, when the uniterruptible power equipment is provided with a battery connected in parallel, this current ripple to the inverter flows into the battery causing a temperature rise therein or deterioration in its durability.
(3) In the case where the inverter is a single-phase inverter, or a three-phase inverter with an unbalanced load, the ripple in the D.C. current input to the inverter becomes much larger than in case of a three-phase inverter with a balanced load, and the frequency of the ripple becomes 2f. In such a case, if a film capacitor is used, because of its capacitance becoming 1/10 or less, the ripple naturally becomes larger. Therefore, in order not to allow the excessive current to flow into the battery, the capacitance of the film capacitor has to be made larger by the sacrifice of economy or a reactor has to be provided in series with the battery, and this makes the circuit complex.
(4) In a voltage type inverter, it is ideal in principle that D.C. power is supplied from a D.C. power source of a low impedance. But, in the case where the film capacitor is used instead of the electrolytic capacitor, the impedance of the D.C. power source becomes higher, and the impedance seen from the output side also becomes higher, and therefore, the regulation of the inverter becomes larger. Specifically, it is a problem with the three-phase inverter that its voltage unbalance becomes larger when connected with an unbalanced load.